1. Field of the Invention
The present invention relates to an engine cooling system in an internal combustion engine.
2. Description of the Prior Art
There is a conventionally known engine cooling system having an electric-powered variable displacement water pump provided adjacent an engine inlet in a cooling water circulation circuit interconnecting an engine body and a radiator. Such an engine cooling system is known, for example, from Japanese Patent Application Laid-open No. 2418/83. Further, an engine cooling system comprising a variable flow rate control valve provided in the cooling water circulation circuit to adjust the amount of cooling water flowing through the engine by controlling the opening degree of the control valve is also known, for example, from Japanese Patent Publication No. 571/89.
To precisely control the operation of the water pump to provide a cooling water temperature suitable for the operational condition of the engine by the cooling system disclosed in Japanese Patent Application Laid-open No. 2418/83, it is desired to effect a feed-back control of the operation of the water pump in accordance with the engine water temperature. In this case, the rapid warming-up of the engine can be achieved by effecting the feed-back control of the water pump in such a manner that the amount of cooling water flowing through the engine body is extremely small during warming-up of the engine. However, a value detected by a water temperature detector is incorrect due to the fact that the amount of cooling water flowing through the engine body during warm-up is extremely small. For this reason, in the engine cooling system using the feed-back control, it is difficult to increase the engine water temperature up to a desirable value, and the engine water temperature may be increased more than necessary, in some cases.
The present inventors have found, such as shown in FIG. 10, that for the purpose of avoiding the generation of engine knocking, it is effective to reduce the difference between an engine inlet water temperature and an engine outlet water temperature. However, in such a feed-back control as described above, it is difficult to optimally control the difference between the engine inlet water temperature and the engine outlet water temperature to avoid the generation of knocking.
In order to shorten the warming-up time and to avoid an over-cooled state, a target value for the engine outlet water temperature may be previously set at a relatively high level, and the opening degree of the control valve may be controlled, so as to bring the engine outlet water temperature to the target value. In the prior art system disclosed in the Japanese Patent Publication No. 571/89, in a normal operational condition, the opening degree of the control valve is controlled in accordance with the engine load and the engine speed so as to bring the temperature of a cylinder wall into a predetermined range, on the one hand, and during warming-up of the engine, the opening degree (the fully closed state) of the control valve is controlled so that the engine outlet water temperature is about 120.degree. C., on the other hand. However, when the amount of the cooling water is controlled with only the engine outlet water temperature, the engine may become overheated. In the above prior art system, the temperature of the cylinder wall is detected directly, and the opening degree of the control valve is corrected by such detected value, so that the temperature of the cylinder wall is maintained within the predetermined range. For this reason, the arrangement of a control circuit is complicated.